This fancy ichneumonid wasp wandered a little on the window frame, a female Tromatobia lineatoria [1] (validation determination). A new garden species.
This species lays her eggs in spider egg sacks [2].

References
1 Nederlands Soortenregister2 FITTON, Michael Geoffrey, et al. Pimpline ichneumon-flies. Hymenoptera, Ichneumonidae (Pimplinae). Pimpline ichneumon-flies. Hymenoptera, Ichneumonidae (Pimplinae)., 1988, 7.1.